In known prior-art devices, electrical leakage currents may be observed to appear through the thin organic layer that is supposed to electrically insulate the two conductive electrodes, which layer is also called the active layer.
These leakage currents are dependent, on the one hand, on intrinsic properties of the active layer (in particular its conductivity, on the presence of electrical traps, on the position of the HOMO-LUMO energy levels with respect to the work functions of the electrodes, or on the morphology of the layer) and, on the other hand, on extrinsic parameters such as parasitic electrical leakage currents.
These parasitic currents are not controlled. They essentially originate from topological defects, i.e. holes or morphological defects, i.e. zones of larger free volume. They are generated during the formation of the active layer.
Thus, the presence of holes in the active layer may lead to the two conductive electrodes short-circuiting locally. Moreover, the zones of different morphology are more propitious to electrical breakdown.
These defects in the active layer may be due to the materials used to form the layer, which take the form of a solution possibly including aggregates, i.e. material poorly dissolved in the solution. They may also result from defects present in the substrate, such as topological defects or peaks in the surface or zones of different surface tensions.
These parasitic leakage currents are very disadvantageous when they occur in organic photodetectors or current-rectifying diodes.
Specifically, in this case, the reverse-leakage and dark current of the diode must be very small (of the order of one nA/cm2). Thus, the slightest electrical leakage through defects in the active layer may lead to this current increasing by several orders of magnitude and the performance of the diode being drastically and irreversibly degraded.
These parasitic leakage currents are also disadvantageous for organic solar cells, but to a lesser extent.
For such a device, the lower the leakage current of the diode, the more the solar cell will be able to respond to a weak illumination.
Thus, solutions for minimizing parasitic leakage currents in the active layer of a stack have already been proposed.
It has in particular been proposed to increase the thickness of the active layer, to filter the solutions before their deposition to form the active layer and to use substrates containing few defects.
However, the proposed solutions have drawbacks.
Specifically, too great an increase in the thickness of the active layer tends, for example, to degrade device performance. This is why the thickness of the active layer is generally about 200-300 nm. Moreover, a filtration requires a solution having a good solubility, this not being the case for all the materials currently available for active layers. In addition, the filtering step is difficult to implement on an industrial scale. Lastly, substrates having few defects are substrates with planarizing layers, which are of a high cost.
Mention may also be made of document FR-2991505, which describes a process for producing a stack of a first electrode/active layer/second electrode allowing parasitic electrical leakage currents to be decreased.
This process firstly consists in depositing a first conductor layer on a substrate, in order to form the first electrode, then an active layer, taking the form of a thin organic semiconductor layer, this layer containing defects.
This process then consists in removing locally, via chemical attack, the conductive first layer, through the defects in the active layer.
A second conductor layer is then deposited on the active layer, to form the conductive second electrode.
Because of the local removal of the conductive layer, level with the defects in the active layer, the two electrodes can no longer make contact and, thus, cannot create an electrical short-circuit through the active layer. Electrical leakage currents are therefore considerably decreased.
This process therefore allows the risks of short-circuits to be decreased.
However, it is not suitable in the case where the conductive second layer is very liquid. Specifically, it is then liable to infiltrate under the organic layer and to once again make contact with the conductive first layer beyond the etched zone under the defect.
In addition, the process requires the conductive first layer to be completely etched away level with the defects in order to prevent a short-circuit. Thus, in order to be effective, the process requires the conductive first layer to be substantially overetched with respect to the dimension of the aperture in the active layer, because, when the conductive second layer is deposited, it infiltrates a little under the active layer and may thus generate a short-circuit.